Hitherto, a water jet propulsion apparatus has been known as disclosed in, for example, Japanese Patent Laid-Open No. 5-270486, with which water is sucked from a suction opening opened in a bottom of a ship, sucked water is pressurized by a horizontal impeller of a pump disposed above a surface of water and water is jet to a position in the rear of a stern of the ship so that the ship is propelled. Another water jet propulsion apparatus has been disclosed in, for example, Japanese Patent Publication No. 7-117076, which incorporates a volute casing disposed horizontally and with which an impeller is rotated to spirally swirl water sucked from a position below the bottom of the ship so as to jet a swirl water flow to a rear portion to the ship.
The water jet propulsion apparatus disclosed in Japanese Patent Laid-Open No. 5-270486, however, has the structure that the impeller of the pump is disposed above the surface of water. Therefore, when the ship starts navigating, the internal portion of a pump casing must be negative pressure to lift water below the surface of water to the position of the impeller. Thus, there is apprehension about difficulty in starting easily.
Since the impeller is disposed apart from the bottom of the ship, a passage in a suction portion of the impeller is too long, a long actual lift to the impeller is required and great resistance is generated in the suction portion. As a result, cavitation takes place when the ship is navigated at high speed.
Since the propulsion apparatus is secured to the ship at the suction and discharge portions to cause the suction portion to be supported at the bottom of the ship and the discharge portion to be supported at the stern, a process for making coincide a main shaft of the impeller and the axis of a drive shaft of a motor with each other cannot easily be performed. A deviation between the two shafts must be absorbed by dint of a play realized by securing a projection portion and the stern to each other such that the somewhat play is provided. If the two axes are connected to each other with an eccentricity, the main shaft disposed horizontally is deflected by dint of the weight of the impeller and vibrations of the motor are transmitted to the main shaft. Therefore, the rotating impeller is brought to the bottom of the pump casing, thus causing the impeller to be worn. Thus, there is apprehension that an adverse influence is exerted on the efficiency of the pump.
The water jet propulsion apparatus disclosed in Japanese Patent Publication No. 7-117076 has the structure that the volute pump casing is disposed horizontally. Therefore, if the ship is separated from the surface of water because of waves and thus air is sucked together with water, air cannot easily be discharged. Thus, eddy currents of air are generated, causing cavitation to take place. As a result, there is apprehension that the propelling performance deteriorates.
The present invention has been achieved to solve the above-mentioned problems, an object of the present invention is to provide a water jet propulsion apparatus which is capable of reducing resistance which arises when water is introduced and cavitation occurring when a ship is navigated at high speed and which can easily be mounted.